The present invention relates to a cathode ray tube used in a television set, a computer display monitor or the like.
When a cathode ray tube is placed in an external magnetic field such as terrestrial magnetism or the like, electrons ejected from an electron gun receive an additional Lorenz force generated by the action of the external magnetic field, so that the trajectory of electrons are deviated and landing points of the electrons are deviated from a normal location on the fluorescent material, as a result, electrons collide (referred to as mislanding). Furthermore, in the cathode ray tube having a shadow mask, the mislanding is also generated by the leaked magnetic flux generated with the magnetization of the mask frame for stretching and fixing the shadow mask.
Materials of conventional mask frames have a small relative magnetic permeability so that the materials are not easily magnetized. In many cases, such mask frame is formed of a material which is not easily brought back to zero magnetization state once it magnetized (a hard magnetic substance), so that the magnetic flux generated as the result of the magnetization of the material is easily leaked to the inside space of the cathode ray tube.
In order to reduce the deviation of the electron beams generated by the external magnetic field and the secondary leakage magnetic field or the like by the external magnetic field as described above, an inner magnetic shield is attached on the inside of the cathode ray tube to reduce the influence from the external magnetic field.
To summarize above description, the cathode ray tube having the conventional shadow mask described above has the following problem.
The conventional mask frame is not sufficiently magnetically shielded by the inner magnetic shield, so that the influence by the leakage magnetic field from the mask frame cannot be ignored.
A group of the present invention has been made in view of the present situation described above, and an object thereof is to provide a cathode ray tube in which the deviation in the trajectory of the electron beams resulted from the leakage magnetic field from the mask frame is reduced.
A group of the present invention has been made to attain the first object, and there is provided a cathode ray tube comprising a bulb whose inside maintained in vacuum, an electron gun provided inside of the bulb to eject electron beams, a fluorescent material which is provided on the inside wall of the bulb and which emits light when electron beams ejected from the electron gun are applied thereto, beam deflection means for deflecting the electron beams so as to scan the surface of the fluorescent material, an inner magnetic shield provided inside of the bulb for decreasing the deviation of the trajectory of the electron beams deflected by the beam deflection means in the external magnetic field, a shadow mask arranged in front of the inside surface of the fluorescent material and a polygon-shaped mask frame for fixing the shadow mask, the frame having a side member constituting each side and a joint portion for joining adjacent side members, wherein the adjacent side members in each joint portion satisfies the following condition;
xcexcr,1xe2x89xa7xcexcr,sxe2x89xa71
where xcexcr,1 represents a relative magnetic permeability of longer side member and xcexcr,s represents a relative magnetic permeability of the other member.
When the concept of the magnetic circuit is introduced, it becomes possible to conveniently analyze the magnetic property in terms of characteristics. Consequently, the explanation below is given by using this concept. Since the mask frame and the shadow mask are normally formed of a magnetic substance, the mask frame, the shadow mask and the inner magnetic shield are considered as a magnetic resistance of the equivalent magnetic circuit. Here, the current in the electric circuit corresponds to the flow of the magnetic flux which flows through the virtual magnetic resistances. The current source in the equivalent electric circuit corresponds to the terrestrial magnetism which is an origin of the flow of the magnetic flux.
The smaller the relative magnetic permeability of the above-mentioned longer member becomes, the larger the magnetic resistance of the above-mentioned longer side member becomes. Consequently, the magnetic flux which flows through the other member having a small magnetic resistance cannot be perfectly absorbed with the result that the magnetic flux which flows through the virtual magnetic resistance of the vacuum space arranged in parallel increases. That is, a larger amount of the flow of the magnetic flux is leaked to the space inside of the mask frame. Consequently, the side member constituting the mask frame and having a longer length can decrease the magnetic field which is leaked to the space of the inside of the mask frame when the relative magnetic permeability is enlarged as compared with the short side member.
Here, the side member having a relative magnetic permeability of 1 or more refers to a non-magnetic substance and a magnetic substance. Furthermore, the magnetic substance is a generic name for a ferromagnetic substance including a hard magnetic substance and a soft magnetic substance, and an anti-ferromagnetic substance. In this specification, the non-magnetic substance refers to a substance having a relative permeability of 1, the hard magnetic substance refers to a substance having the relative permeability of bigger than one 1 and smaller than 100. The soft magnetic substance refers to a substance having a relative permeability of 100 or more.
Furthermore, the side member refers to a member having a part constituting one side of the mask frame. Consequently, the side member may not be a member having only a part constituting one side of the mask frame. Furthermore, the polygon-shaped mask frame refers not only to a frame having an external configuration forming a polygon but also to a frame or the like having a part projecting out of the frame.
Furthermore, the beam deflection means may be an electric field deflection means for deflecting the electron beams by the action of the electric field and the deflection means may be the magnetic field deflection means for deflecting electron beams by the action of the magnetic field. Generally, the magnetic field deviation means is used.
Since the display device such as a television set, a computer display or the like generally has a parallelogram-shaped display screen, preferably, the mask frame comprises a pair of short side members and a pair of long side members for stretching and fixing the shadow mask, and an overlapping surface of the pair of short side members and the pair of long side members is formed in a parallelogram-like configuration formed on the same planar surface. More preferably, the mask frame is a square-shaped or a rectangular-shaped mask frame. With the square-shaped or the rectangular-shaped mask frame, it becomes easy to manufacture a mechanically strong frame.
It is known that in the rectangular-shaped mask frame, the magnetic field is leaked from the end portion of the long side member and the joint portion of the long side member and the short side member. Since the magnetic flux from the end portion of the long side member is hardly leaked to the inside space of the inner magnetic shield, this magnetic flux does not affect the increase in the deviation in the beam so much. Furthermore, an influence of the leakage magnetic field from the end portion of the long side member becomes smaller with a reduction in a distance of the inner magnetic shield to the rectangular-shaped mask frame. However, the magnetic field is leaked to the inside space of the inner magnetic shield from the joint portion of the short side member and the long side member, which largely contributes to the deviation in the electron beams. The terrestrial magnetic field is absorbed by the open portion of the inner magnetic shield and flows to the mask frame magnetizing the mask frame. When the relative magnetic permeability of the short side member is different from the relative magnetic permeability of the long side member, so that if a considerable difference is present in the magnetic resistances of the two, a larger amount of magnetic flux flows to the side member having a smaller magnetic resistance. On the other hand, only a small amount of magnetic flux can flow through the side member having a high magnetic resistance. Consequently, the flow of the magnetic flux will inevitably flow out at the joint portion of the short side member and the long side member. Consequently, when the relative magnetic permeability of the long side member is set to be larger than the relative magnetic permeability of the short side member, the flow of the magnetic flux in the short side member flows into the long side member, so that the leakage of the magnetic field into the inside space of the inner magnetic shield can be decreased.
In the case of the parallelogram-shaped mask frame in which the end of the pair of the long side members in the longitudinal direction is projected in an outward direction from the joint portion while the end of the short side member in the longitudinal direction is not projected in an outward direction, a strong joint by welding can be made possible with the result that the frame having an extremely strong mechanic strength can be manufactured. Besides, the flow of the magnetic flux is concentrated on the end portion of the long side member which generally has a high relative magnetic permeability. However, the end portion is distanced from the mask frame, so that the influence of the leakage magnetic field from the end portion inside of the mask frame becomes small.
For a pair of the short side members and a pair of the long side members, the hard magnetic substance and the soft magnetic substance, the hard magnetic substance and the hard magnetic substance, the non-magnetic substance and the soft magnetic substance, the non-magnetic substance and the hard magnetic substance and the non-magnetic substance and the non-magnetic substance can be used.
When the relative magnetic permeability of the pair of the short side members is ⅓ or less of the relative magnetic permeability of the pair of long side members, the leakage of the magnetic field to the inside of the mask frame can be effectively decreased. Furthermore, in the similar manner, when the product of the relative magnetic permeability of the pair of short side members and the cross sectional area of the short side members in the joint portion is ⅓ or less of the product of the relative magnetic permeability of the long side members and the cross sectional area of the long side member in the joint portion, the leakage of the magnetic field into the inside space of the mask frame can be decreased.
In the case where the soft magnetic substance is used as the long side member, it is possible to form a mask frame which has a mechanical strength required for stretching and fixing the shadow mask and which decreases the leakage magnetic field inside of the mask frame when the long side member is a soft magnetic substance having a relative magnetic permeability of 200 or less.